In recent years, development has been under way of a lawn mower of a type where a plurality of cutter blades is mounted on a blade disk drivable by a motor to rotate in a substantially horizontal plane. Such a lawn mower is popularly used as an unmanned self-propelled lawn mower (so-called robotic lawn mower), one example of which is disclosed in European Patent Application Publication No. 2656720 (hereinafter referred to as “the relevant patent literature”).
The lawn mower disclosed in the relevant patent literature includes: a rotation shaft extending in a vertical or up-down direction and drivable by a motor; a blade disk mounted on the rotation shaft; and a plurality of cutter blades mounted on the blade disk. The plurality of cutter blades cuts grass on a lawn by being rotated in a substantially horizontal direction.
The blade disk, and hence the plurality of cutter blades, rotates at high speed in order to cut grass. When grass is being cut by the plurality of cutter blades, a relatively great external force is transmitted from the plurality of cutter blades to the blade disk, and thus, there may occur a so-called waving phenomenon where the blade disk waves in the axial direction of the rotation shaft due to the external force. Such a waving phenomenon would cause vibrations of the blade disk that would in turn cause unwanted operating sound. It is preferable that production of such operating noise be minimized in order to maintain in good condition environment around the area to be mowed. However, providing a large-scale sound deadening device on the lawn mower would lead to a significant cost increase of the lawn mower.